Avoid a crash when unencoding session data for the db backend. This is required

because some configurations of MySQL (with utf8_bin collation) will return
bytestring, rather than unicode data, which was causing problems previously.

Refs #8340.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8507 bcc190cf-cafb-0310-a4f2-bffc1f526a37